Usual magnetic fluids are prepared with the addition of a surfactant such as a oleate salt or a dodecylamine in order to make the superfine particles of a magnetic metal oxide or magnetic metal stably existing in a liquid such as water or an oil. Since usual magnetic fluids are poor in chemical stability and have high toxicity, trials to apply them to medical treatment have scarcely been made.
In order to obviate these drawbacks, complexes of dextran with a magnetic iron oxide have been proposed, and for example, Japanese Patent Publication No. 13521/1984 (U.S. Pat. No. 4,10l,435) discloses complexes of dextran or a modified dextran obtained by treating dextran with sodium hydroxide and a magnetic iron oxide having a size of 3 to 20 nm. Further, U.S. Pat. No. 4,452,773 discloses magnetic iron-dextran microspheres of a molecular complex structure such that magnetic iron oxide particles having a colloidal size was coated with dextran molecules. Although these complexes are useful in the form of aqueous sol, they still have problems to be improved particularly for use in medical treatment.
As a result of intensive studies, the present inventors found that complexes of a carboxyalkyl ether of a polysaccharide with a magnetic metal oxide have improved properties in several points, and came to complete the invention.